1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to an electromagnetic transducer having a diaphragm or cone which is asymmetric, meaning that the ID of the cone is not at the geometric center of the OD of the cone. The ID or inner diameter refers to the location, typically but not always a hole, where the bobbin attaches to the diaphragm. The OD or outer diameter refers to the outer perimeter where typically the surround attaches to the diaphragm.
2. Background Art
FIG. 1 illustrates a conventional audio speaker 10 such as is known in the prior art. The speaker includes a motor assembly 12 coupled to a diaphragm assembly 14 by a frame 16. The motor assembly includes a magnetic air gap 18 over which magnetic flux flows. The diaphragm assembly includes an electrically conductive voice coil 20 which is rigidly attached to a bobbin or voice coil former 22. The voice coil is suspended within the magnetic air gap to provide mechanical force to an acoustical radiating member 24, often termed a diaphragm or cone, which is coupled to the bobbin. When an alternating electric current is passed through the voice coil, the voice coil moves axially in the air gap, causing the diaphragm to generate sound waves. The diaphragm assembly further includes two suspension components which serve to keep the bobbin and diaphragm centered and aligned with respect to the motor assembly, while allowing axial movement. A damper or spider 26 is coupled to the bobbin and the frame, and a surround 28 is coupled to the diaphragm and the frame. A dust cap 30 seals the assembly and protects against infiltration of dust particles and other stray materials which might contaminate the magnetic air gap and thereby interfere with the operation or quality of the speaker.
The motor assembly has an axis Am typically understood to be at the axial center of the magnetic air gap in which the voice coil rides. The diaphragm has an OD or outer perimeter which has a geometric center or axis Aod. It is the same distance OD1 from the axis Aod to a first point on the OD and to a second point on the OD, which two points are radially opposite each other. The diaphragm may be axisymmetric, in the case of e.g. a round 6″ speaker. Alternatively, the diaphragm may be bilaterally symmetric, in the case of e.g. an elliptical 6×9 speaker. Other diaphragm OD shapes are known in the art, as well. The diaphragm also has an ID or inner perimeter which has a geometric center or axis Aid. It is the same distance ID1 from the axis Aid to a first point on the ID and to a second point on the ID, which two points are radially opposite each other. In nearly all cases, speakers use a cylindrical bobbin and a circular ID, but a few exceptions are known. The spider has a center or axis of suspension Asp, and the surround has an center or axis of suspension Asu.
As shown in FIG. 1, virtually all known speakers are constructed such that the motor axis Am, the axis Aod of the OD, the axis Aid of the ID, the axis of suspension Asp of the spider, and the axis of suspension Asu of the surround, are all coaxial with one another.
Ordinarily, in most engineering applications it is desirable to achieve symmetry. However, in audio applications, symmetry has some disadvantages. For example, a symmetric cone exhibits the same breakup modes in all radial segments, as each radial segment has the same shape, size, mass, etc. as the others. As another example, a symmetric speaker exhibits equal diffraction characteristics and cone/edge junction modes at all radial segments.
FIGS. 2 and 3 are copied from U.S. Pat. No. 5,022,488 “Transducer Enclosure” issued Jun. 11, 1991 to William House and assigned to Harman International. The House patent teaches a speaker 31 having an asymmetric diaphragm 33. That inventor was addressing a completely unrelated problem, that of fitting two speakers 35, 37 into a single cabinet 39 with separate pressure venting for each. He appears to have moved the woofer's motor structure 41 away from the center of the woofer's diaphragm 33 merely for the purpose of providing physical space for the tweeter 37 to fit in front of a portion of the woofer's diaphragm, and not to have recognized any other benefits from the asymmetry. Indeed, the patent states that “it is not necessary for diaphragm [of the woofer] to be asymmetric, nor for diaphragm [of the tweeter] to be symmetric, nor for either transducer to be a diaphragm type at all.” (col. 3 lines 57–60, reference numbers omitted, bracketed text added). It is noteworthy that the woofer diaphragm is an inverted cone, rather than a conventionally oriented cone with its bell facing outward. This was clearly done to make still more room for the tweeter within the design constraint of gaining “the benefits of shallow loudspeaker mounting” (col. 1 line 11).